1. Field of the Invention
This invention relates to methods for etching metal in semiconductor fabrications, and more particularly, to a method for etching metal with increased etching selectivity.
2. Description of Related Art
In semiconductor fabrications, etching is an essential step which is used to remove unwanted portions of either a metal layer, a semiconductor layer, or a photoresist layer during the photolithographic process. There are two primary types of etching: wet etching and dry etching. Wet etching is conducted by using chemical solutions and is therefore isotropic to the layer being etched since it often causes undercuts beneath the layer being etched. By contrast, dry etching is conducted using plasma and therefore is anisotropic to the layer being etched making dry etching more suitable for use in high precision mask processes.
In designing and use of the dry etching, it is desired that the etching be primarily directed to the downward direction and minimally to the lateral direction so that the transferred pattern on the wafer can be substantially the same in size as the original pattern on the mask and an undercut can be avoided. In other words, the ratio of the downward etching rate to the lateral etching rate should be as large as possible. The ratio of the etching rate on the layer aimed to be etched to the etching rate on the nearby layer is referred to as "etching selectivity". However, since the anisotropic characteristic of dry etching is owing to the bombardment of particles on the wafer, the etching will not only take place on the layer aimed to be etched, but also on the overlaying photoresist layer. The etching removal of part of the photoresist layer will cause a deviation in the size of the pattern being defined by the photoresist layer, thus giving a poor etching selectivity that is even lower than that achieved by wet etching. A large value for the etching selectivity indicates that the etching mainly takes place on the layer aimed to be etched and minimally on the nearby layers. Therefore, although dry etching can give high precision to the pattern being transferred, it nevertheless gives poor etching selectivity. The following will depict this drawback illustratively.
FIGS. 1A through 1C are sectional diagrams depicting the steps involved in a prior art method for etching metal. Referring first to FIG. 1A, a metal layer 12 is deposited over an insulating layer 10. The insulating layer 10 is, for example, a layer of borophosphosilicate glass (BPSG).
Referring next to FIG. 1B, a photoresist layer 14 is formed over the metal layer 12. The photoresist layer 14 is selectively removed by a photolithographic process so as to define a desired pattern.
Referring further to FIG. 1C, a dry etching process is conducted on the wafer so as to remove the exposed portions of the metal layer 12 that are uncovered by the photoresist layer 14. Through this process, the exposed portions of the metal layer 12 as well as part of the photoresist layer 14 will be etched away. After that, the photoresist layer 14 is removed and the remaining wafer is as that shown in FIG. 1C.
By the foregoing etching process, the etching rate to the metal layer 12 is relatively low compared to the etching rate to the photoresist layer 14, thus giving a very poor metal-to-photoresist etching selectivity. In order to allow the transferred pattern to be precise in size, a solution is to increase the thickness of the photoresist layer. However, this will equally reduce the size of the process window for the photolithographic process.
Another solution is to use high-current implantor (HI-implantor) to diffuse arsenic (As) ions in the photoresist layer so that the photoresist layer can be more resistant to etching. This can increase the metal-to-photoresist etching selectivity. However, a drawback to this solution is that, during the implantation, the bombarding ions will strike metal ions away from and off the metal layer, thus causing the implantor to be polluted by these metal ions. There exists, therefore, a need for a new and improved method for etching metal which not only increases the metal-to-photoresist etching selectivity, but also prevents the implantor from being polluted by metal ions.